1. Field of the Invention
This invention relates to an improved process for the surface treatment of polymeric compositions and is more particularly concerned with a process for removing surface tack from articles fabricated from radiation-sensitive polymerizable resins and with the treated articles so prepared.
2. Description of the Prior Art
It is well-recognized in the art that resin compositions which have been polymerised and cured by exposure to actinic or thermal radiation frequently exhibit a stickiness or surface tack which is attributable to incomplete reaction of the reaction components even after curing. This is particularly true of relief printing plates and like articles which are prepared by selective exposure (i.e. via a negative transparency) of parts of the surface of the radiation-sensitive polymerizable resin to appropriate radiation followed by solvent etching of unpolymerized resin to form the relief image. The portions of the resulting surface from which the unreacted resin has been removed by the solvent etching are particularly prone to exhibit tack.
The surface tack exhibited by articles such as those described above causes problems arising from the tendency of the articles to pick up dust, particularly paper dust in the case of relief printing plates, which can interfere with the efficient use of the articles in question. In addition the articles will tend to adhere to each other if stored in piles for any significant length of time.
Various means of overcoming the problem of surface tack have been described. Thus, it has been proposed to provide a thin coating of rubber latex or other such materials on the surface exhibiting tack. Such coatings suffer disadvantageous such as the tendency to lose their effectiveness after extended periods of use, particularly where the treated article is a relief printing plate. Further, such coatings tend to blur the sharpness of any relief image which has been developed on the surface prior to the coating.
U.S. Pat. No. 4,234,676 (Hein et. al.) describes a process for reducing surface tack on a relief printing plate derived from a photopolymerizable resin composition by the inclusion in said composition of a polythiol.
U.S. Pat. No. 4,400,460 (Fickes et. al.) describes the post-treatment of a relief printing plate derived from a photosensitive polymer by the steps, in either order, of post-exposing the plate to a source of actinic radiation and contacting the plate with an aqueous solution containing a source of bromine. U.S. Pat. No. 4,451,553 (Fickes et. al.) describes a closely related post treatment in which the plate is exposed to the action of two successive aqueous solutions the first of which contains a source of bromine or iodine and the second of which contains a source of chlorine.
U.S. Pat. No. 4,415,654 (Pohl) describes post-treatment of a relief printing plate derived from a photosensitive polymer by immersing the cured plate in a dilute aqueous solution of a persulfate salt (e.g. sodium) which solution also contains sufficient of a carboxylic acid (e.g. acetic acid) to provide a pH of about 1.5 to 2.5, and exposing the immersed plate to actinic radiation.
U.S. Pat. No. 4,202,696 (Takahashi et. al.) discloses a method of removing tack from the surface of a free radical polymerized resin composition by impregnating the surface with an organic ketone and exposing the impregnated surface to the action of actinic rays. This procedure, when employed to treat a relief printing plate derived from a photosensitive resin composition, has been found to cause undesirable swelling of the plate.
I have now found that surface tack on relief printing plates and related articles prepared from radiation-sensitive polymeric compositions, can be eliminated, or reduced to acceptable levels, by a simple and effective post-treatment which will be described in detail hereinbelow.
Accordingly, it is an object of the invention to provide an effective and economically advantageous method of eliminating or satisfactorily reducing the surface tack exhibited by an article fabricated from a radation-sensitive polymerizable resin composition. It is another object of the invention to provide an article fabricated from such a polymerizable resin composition which article has a surface substantially free from tack. It is yet a further and particular object of the invention to provide a process for eliminating or satisfactorily reducing the surface tack on a relief printing plate fabricated from a photosensitive resin composition and to thereby provide relief printing plates which can be utilized to make a large number of consecutive impressions without any recurrence of surface tack or deterioration because of exposure to printing inks, solvents and the like used in the printing process.